1. Field of the Invention
This invention relates to a process and apparatus for growing single crystals of alpha-aluminum orthophosphate and alpha-gallium orthophosphate.
2. Description of the Prior Art
Alpha-aluminum orthophosphate (berlinite) and alpha-gallium orthophosphate (GaPO.sub.4) are among several alpha-quartz isomorphs that have for decades been synthesized for research purposes. An attempt to grow large single crystals of berlinite began after World War II, in an effort to find new piezoelectric crystals for frequency control applications. The project ended a few years later, because success was achieved in quartz crystal growth and because quartz was considered superior for the piezoelectric devices known then. Specifically, it was concluded that berlinite had a lower Q and lower coupling coefficient than quartz. Furthermore, tests on both X and Y cuts of berlinite plates, showing a negative frequency drift with increasing temperature, indicated that there was little chance of finding a zero temperature cut similar to the AT cut of quartz.
Interest in berlinite was renewed in 1976, when Barsch and Chang found that berlinite does have temperature-compensated cuts and that the coupling coefficient for surface acoustic wave (SAW) devices can be four times greater than for quartz.
Several processes for preparing berlinite have been reported in the technical literature (W. Jahn et al., Chem. Erde 16, 75 (1953); J. M. Stanley, Ind. Eng. Chem. 46, 1684 (1954); E. D. Kolb et al., J. Crystal Growth 43, 313 (1978)).
Generally, two types of processes are described, the slow-heating method and the temperature gradient method. In a typical process, seed crystals are suspended near the bottom of a vertical autoclave are nutrient powder is suspended in a basket at the top. Both seed and nutrient are immersed in concentrated phosphoric acid. In one variation of the process, the temperature is raised from about 150.degree. C. to about 190.degree. C. at about 2.degree. C./day. In another variation, a temperature gradient is maintained in the autoclave.
Recently, Drafall and Belt, of the Rome Air Development Center, have developed a combination process in which a constant temperature gradient is maintained in the autoclave as the temperature is raised about 10.degree. C. in about 14 days. The quality of the crystals they produce, however, was not satisfactory for device applications (RADC-TR-80-73, Final Technical Report, March, 1980).